OK the first thing you have to check is to make sure the compressor is turning ON when you turn on the AC switch (depress the EC switch and the light should go OFF).
If the EC light does not go OFF, it means the compressor is not ON, because of three conditions... 1. there is not enough Freon (R134a) in the system, 2. the temperature parameters will not allow the compressor to turn on, 3. bad thermo switch on the receiver dryer.
Question... Was the AC always ice cold, but suddenly stopped working because the EC light was always turned ON... OR was the AC cold temp gradually getting less COLD over a period of weeks?
I am going to take a guess that it was the first case... since you said the EC light was always ON. I think there is enough freon in the system to cool it down but you have to check it with a thermometer in the center vent to be sure if it needs more freon added.
Go do a search on this forum or the W124 forum for AC EC light always ON...
It will bring up a thread where it explains how to reset the AC temp parameters using the AC front control panel; the object is to get the EC light to turn OFF meaning the compressor will turn on so you can check the degree of cooling.
Assuming the problem is not a Thermo switch (on the receiver dryer), and you follow the procedure correctly, the compressor should turn ON.... set the temp to MAX cold and MAX Fan speed... stick a 6 inch dial thermometer iinto the center vent and watch the temp scale. Depending on the ambient temperature (not a hot day) the dial thermometer should read about 50 deg. F around 1500 -2000 rpm (yes, give it a little GAS). If you read only 60 deg R. then add a can of R134a while watching the temp.
The BLACK cap is the LOW (suction) side where the freon is added... The high side is a larger valve so the freon can/hose will not fit on the valve. You should have the engine and compressor running before connecting the freon can as the LOW side should be around 30-45 psi while the HIGH side will be around 150-180 psi.
I would recommend not using the plastic cap/valve that comes in the AC kit and attaches to the freon can, but buy from Kragen the metal brass freon can valve for $6.95 which will screw on the can straight (not angled) and allow the pin valve to puncture the can without leakage.
That should be enough to get you started for now... let us know what happens.
Good Luck..
